In the light source of a conventional photographic enlarger, light is emitted continuously from a light emitter such as a halogen lamp that emits light of a constant intensity, and the duration of the light emission is controlled to control the amount of the emitted light and accordingly the amount of exposure for the photographic printing. In practice, however, it is difficult to keep constant the intensity of the light emitted from such a light source. To keep the intensity constant, the light source requires a power source stabilizer and the like and becomes complicated, cumbersome and thereby expensive. Moreover, a halogen lamp is bulky itself and has low efficiency of conversion from electricity to light. Of the electric energy applied to the lamp, the portion that is not converted to light, is converted to heat so that a large amount of heat is produced during the energization of the lamp. To avoid the effects of such heat, the light source device of the conventional enlarger requireds a cooling device such as a heat diffusing plate, a wind fan or the like and becomes inherently large in size.
Especially in the case of a color enlarger for printing color photographs, three light source lamps are employed for the exposure with three primary color lights of blue, green and red or otherwise a single light source lamp is coupled with three kinds of color filters or dichroic filters of blue, green and red such that the light from the lamp is incident on a film through the filters. In any case, the light source device becomes more cumbersome due to the increase in the number of lamps, or the provision of filters and a mechanism for the insertion and withdrawal of the filters and for the control of the amount of the insertion or withdrawal. Additionally, the amount of exposure must be controlled more accurately in color printing than in the case of monochromatic photography printing.
On the other side, a photo-engraving camera is known which employs a pulse light emitting light source such as a Xenon tube and in which the total amount of light emission i.e. the amount of exposure is controlled by controlling the number of the emissions of the pulse light source on the presumption that all the light amount at each emission of the pulse light source is equal. The light source such as the Xenon tube has various advantageous features. That is, it is comparatively small in size as may be understood from the fact of being used as a flash device for use with a photographic camera. Further, the light source has good efficiency of electricity-to-light conversion and is a so-called heatless (or heat-free) light source. Accordingly, compactness of the power source may be attained if such a light source as the Xenon tube is adopted in a photographic enlarger. In the case where the amount of exposure is controlled by adjusting the number of light emissions as in the photo-graving camera, however, the amount of light at each emission must be made small resulting in an increase of the number of light emissions and accordingly an increase in the exposure time for obtaining a larger amount of exposure, if it is intended to improve the resolution of the exposure adjustment i.e. to reduce the unit change of the exposure amount. Additionally, the amount of pulse light emitted from the aforementioned light source varies in practice and it is likely to occur that the amount of an actual exposure differs from the amount estimated from the number of light emissions.